This invention relates to a method of making an optical waveguide preform. More specifically, the method of the present invention is useful for making low loss optical waveguides, especially waveguide fibers having a segmented core profile.
Optical fibers having refractive index profiles such as W-profiles, segmented core profiles, and the like possess desirable dispersion characteristics. See U.S. Pat. Nos. 4,715,679 and 5,031,131 for teachings of various kinds of dispersion modified optical fibers. Fibers having these kinds of refractive index profiles have often been made by chemical vapor deposition (CVD) processes such as plasma CVD processes that are capable of forming single-mode fibers the cores of which include layers of different refractive indices. Such processes produce relatively small preforms. It is advantageous to form dispersion modified optical fiber preforms by outside vapor deposition (OVD) processes which produce relatively large preforms or draw blanks to decrease the cost of making the fiber.
A typical OVD process for forming such fibers is disclosed in U.S. Pat. No. 4,629,485. In accordance with that patent, a germania-doped silica rod is formed and stretched to decrease its diameter. A piece of the rod is used as a mandrel upon which pure silica glass particles or soot is deposited. The resultant composite structure is heated in a consolidation (drying and sintering) furnace through which a fluorine-containing gas flows. The soot is therefore doped with fluorine and sinters on the rod. One or more additional layers of glass are formed on the outer surface of the fluorine-doped silica layer to form a blank from which a fiber can be drawn.
When soot is sintered in accordance with the aforementioned method, whereby fluorine is supplied to the porous preform solely by way of the fluorine-containing muffle gas, the fluorine concentration (as measured by the .DELTA. of the fluorine-containing layer) is not sufficient to provide certain desirable optical characteristics. The typical fluorine concentration achieved with muffle gas doping provides a -0.4% .DELTA. when CF.sub.4 is the fluorine-containing constituent. The maximum delta value for CF.sub.4 produced by the above-described process is -0.5% .DELTA..
As used herein, the term .DELTA..sub.a-b, the relative refractive index difference between two materials with refractive indices n.sub.a and n.sub.b, is defined as EQU .DELTA..sub.a-b =(n.sub.a.sup.2 -n.sub.b.sup.2)/(2n.sub.a.sup.2) (1)
For simplicity of expression, .DELTA. is often expressed in percent, i.e. one hundred times .DELTA.. In this discussion, n.sub.a is the refractive index of the fluorine-doped glass and n.sub.b is the refractive index of silica.
When a fluorine-doped silica tube is collapsed onto a germania-doped silica rod, or when a germania-doped silica tube is collapsed onto a fluorine-doped silica rod, it is extremely difficult to achieve a satisfactory interface between those two members. This is so because the interface typically contains many seeds, and much of the resultant preform or blank produces unusable optical waveguide. Such seed formation is less prevalent when members formed of other glass compositions such as a fluorine-doped silica tube and a pure silica rod are fused to form a preform.
U.S. Pat. No. 4,675,040, discloses inserting a core glass rod made of pure silica into a soot tube of cladding material made of pure silica doped with fluorine and sintering the core/clad structure to fuse the cladding over the pure silica core. U.S. Pat. No. 4,668,263 discloses a method for collapsing a silica tube having a fluorine-doped inner layer onto the surface of a silica rod. In accordance with that patent the collapse step is accomplished by rotating the tube and heating it with the flame from a longitudinally traveling burner. That technique could not be employed to make dispersion modified fiber designs of the type that utilize the entire fluorine-doped tube, including the outer surface, as part of the core region or light propagating region of the fiber. The reason for this is that, because the flame wets the glass, i.e. introduces hydroxyl contamination, the resultant fiber would be rendered unsuitable for operation at wavelengths where attenuation due to hydroxyl ions is large. A further disadvantage of this method concerns the temperature of the flame, which is not lower than 1900.degree. C. At such high temperatures, control of the process becomes difficult. The axis of the preform can become non-linear or bowed. If the core rod is a soft glass such as a germania-doped glass, the rod can become softer than the tube; this can result in an out-of-round core or a core that is not concentric with the outer surface of the resultant fiber.
U.S. Pat. No. 4,846,867 discloses a method for collapsing a fluorine-doped silica tube onto the surface of a silica rod. Prior to the tube collapse step, a gas phase etchant is flowed through the gap between the rod and tube while the tube is heated by a flame. In the specific examples, wherein SF.sub.4 is the etchant, a gaseous mixture of SF.sub.6, Cl.sub.2 and oxygen (ratio 1:1:6 by volume) is introduced through a gap between the rod and the tube. Such a gaseous mixture removes glass from the treated surfaces of the rod and tube, thus forming new surfaces at the rod/tube interface. The chlorine is present in an amount sufficient to remove water generated by the fluorine-containing etchant. The outer surface of the resultant preform is thereafter coated with silica soot particles that are dried, doped with fluorine and then sintered to form a blank from which an optical fiber is drawn. The flame that was directed onto the tube during the gas phase etching step introduces water into the outer surface of the tube. The attenuation of the fiber resulting from that water is high. The attenuation at 1380 nm for one example is 30 dB/km which is attributed to contact of the oxyhydrogen flame with the preform.
Copending U.S. patent application Ser. No. 08/795,687, filed on Feb. 5, 1997, entitled "Method of Making Optical Fiber Having Depressed Index Core Region," discloses a method for inserting a germania doped silica glass rod into a fluorine doped silica glass tube to form an assembly and consolidating the assembly to form a seed free interface. The tube may be overclad with cladding material such as pure silica. It has been discovered that while this method avoids a seed free interface between the consolidated germania-doped silica and fluorine-doped silica interface, it is difficult to control attenuation increases due to hydrogen and heat aging in fibers drawn from preforms made by this method. As used herein, the term "hydrogen aging" refers an attenuation increase in an optical waveguide that has been exposed to an atmosphere containing hydrogen at a certain concentration, pressure and temperature. The term "heat aging" refers to an attenuation increase exhibited by an optical waveguide that has been exposed to heat.
In view of the disadvantages discussed above, it would be desirable to provide a method for producing a segmented core optical waveguide preform that allowed the entire light active region of waveguide preform blank to be dried from the inside of the blank. In addition, it would be particularly advantageous to provide a dispersion modified optical waveguide that had low attenuation and exhibited minimal or no attenuation increase due to heat or hydrogen aging.